diff --git a/src/plugins/monofile.ts b/src/plugins/monofile.ts
index 84fb054d..59eba8bd 100644
--- a/src/plugins/monofile.ts
+++ b/src/plugins/monofile.ts
@@ -918,7 +918,7 @@ async function handleAssessments(node: Element): Promise<void> {
     }
 
     // Check if it's a letter grade
-    if (letterGradeMap.hasOwnProperty(trimmedGrade)) {
+    if (Object.prototype.hasOwnProperty.call(letterGradeMap, trimmedGrade)) {
       return letterGradeMap[trimmedGrade]
     }